1. Field of the Invention
The invention relates to an optical waveguide device and, for example, includes an optical waveguide device that is used in an optical modulator in which an optical waveguide is formed on a dielectric substrate to perform optical modulation.
2. Description of the Related Art
An optical modulator is widely used as an optical waveguide device for optical communication. One of configurations of the optical modulator includes an optical waveguide device in which an optical waveguide is formed on a substrate, and external modulation is performed to change the amount of absorption of light in the optical waveguide by a voltage applied to the optical waveguide to thereby convert an electric signal into an optical signal.
FIG. 27 is a view that illustrates an optical modulator. FIG. 28 is a cross-sectional view of the optical modulator. In the optical modulator 100, a metal layer, such as Ti (titanium), is formed on a portion of a crystal substrate 101 that uses LiNbO3 (or LiTaO2) and thermally diffused or patterned, and is then proton exchanged in benzoic acid to form a Mach-Zehnder interferometer-type optical waveguide 110.
The optical waveguide 110 is formed of an input waveguide 111, concurrent waveguides 112a and 112b, and an output waveguide 113. The optical waveguide 110 constitutes a Mach-Zehnder interferometer. A signal electrode 102 is provided on the concurrent waveguide 112a, and a ground electrode 103 is provided on the concurrent waveguide 112b. Thus, a coplanar electrode is formed.
When a Z-cut substrate is used, to utilize a variation in refractive index due to an electric field in a Z direction, as illustrated in FIG. 28, the electrodes are arranged immediately above the waveguides and patterned. In addition, in order to prevent light that propagates in the concurrent waveguides 112a and 112b from being absorbed by the signal electrode 102 or the ground electrode 103; a buffer layer 104 is provided between the crystal substrate 101 and the signal electrode 102 and ground electrode 103.
When the optical modulator 100 is driven at a high speed, the termination of the signal electrode 102 and the termination of the ground electrode 103 are connected by a resistor R0 to serve as a traveling-wave electrode, and a microwave signal is applied from an input side. At this time, the refractive indices of the concurrent waveguides 112a and 112b respectively change like +Δna and −Δnb, and a phase difference between the concurrent waveguides 112a and 112b changes. In accordance with the phase difference, the intensity of an output that is obtained by interference of the outputs of the concurrent waveguides 112a and 112b changes, and an intensity-modulated signal light is output from the output waveguide 113.
In addition, by changing the cross-sectional shapes of the electrodes, the effective refractive index of a microwave that propagates in the signal electrode 102 is controlled, and the speed at which light propagates in the concurrent waveguides 112a and 112b is matched with the speed at which a microwave propagates in the signal electrode 102. Thus, it is possible to obtain high-speed optical response characteristics. Note that the commercial optical modulator 100 is generally driven at a bit rate of 10 Gb/s or 40 Gb/s per channel.
On the other hand, a device that applies the optical modulator 100 includes an RZ modulator that generates an RZ (Return to Zero) optical signal.
FIG. 29 is a view that illustrates the RZ modulator. The RZ modulator 50 has a configuration such that two Mach-Zehnder interferometer-type optical waveguides are connected by a folded waveguide 53.
The optical waveguide is formed of an input waveguide 51, concurrent waveguides 52a-1 and 52b-1, the folded waveguide 53, concurrent waveguides 52a-2 and 52b-2, and an output waveguide 54. Two Mach-Zehnder modulators 5a and 5b are connected by the folded waveguide 53.
A signal electrode 55-1 is provided on the concurrent waveguide 52a-1, and a ground electrode 56 is provided on the concurrent waveguide 52b-1. A signal electrode 55-2 is provided on the concurrent waveguide 52a-2, and a ground electrode 56 is provided on the concurrent waveguide 52b-2.
FIG. 30 is a view that illustrates the folded waveguide 53. The folded waveguide 53 is formed of straight waveguides 53a at straight portions and an arc-shaped curved waveguide 53b having a small radius of curvature. A small optical loss due to radiation is required for the curved waveguide 53b. 
Here, when continuous light enters the input waveguide 51 of the Mach-Zehnder modulator 5a, the continuous light is Y-branched and then travels in the concurrent waveguides 52a-1 and 52b-1. At this time, an NRZ (Non Return to Zero) electric signal is input to the signal electrode 55-1 to drive the Mach-Zehnder modulator 5a. Thus, an NRZ optical signal is generated at a multiplexing portion of the Mach-Zehnder modulator 5a. 
In addition, the NRZ optical signal enters the Mach-Zehnder modulator 5b through the folded waveguide 53, and is Y-branched, and then propagates through the concurrent waveguides 52a-2 and 52b-2. At this time, a clock electric signal is input to the signal electrode 55-2 to drive the Mach-Zehnder modulator 5b. Thus, an RZ-modulated optical signal is generated at a multiplexing portion of the Mach-Zehnder modulator 5b, and it is possible to obtain an RZ optical signal from the output waveguide 54.
In this manner, by connecting the two Mach-Zehnder modulators 5a and 5b through the folded waveguide 53, the long Mach-Zehnder modulators 5a and 5b may be arranged concurrent to each other. Thus, it is possible to implement a compact device structure.
As an existing optical modulator having a folded waveguide, there is a technique for suppressing a radiation loss that occurs in the folded waveguide by providing a groove on the outer peripheral side of the folded waveguide, which is, for example, described at paragraphs [0012] and [0013] and in FIG. 1 in Japanese Unexamined Patent Application Publication No. 2004-287093 ('093 document).
Ti is diffused on the surface of a substrate at a high temperature higher than or equal to 1000° C. to increase the refractive index of a metal portion as compared with the surrounding portions. Thus, an optical waveguide is formed to confine light while allowing the light to propagate.
A method of manufacturing an optical waveguide generally employs Ti diffusion or proton exchange, which can reduce a propagation loss. Even the optical waveguide manufactured by these methods does not sufficiently confine light, and there is a drawback that a radiation loss occurs at the curved portion of the optical waveguide.
For example, in the above RZ modulator 50, when light propagates in the folded waveguide 53, particularly, in the curved waveguide 53b that is largely curved (having a smaller radius of curvature), radiation of light toward the outside of the curved waveguide 53b remarkably appears.
Thus, in the configuration described in '093 document, a groove is recessed in the substrate on the outer peripheral side of the folded waveguide 53 to suppress a radiation loss. FIG. 31 is a view that illustrates a configuration that a groove is provided on the outer peripheral side of the folded waveguide 53. FIG. 32 is a cross-sectional view of the folded waveguide 53 and the groove.
The substrate on the outer peripheral side of the folded waveguide 53 is recessed by etching, or the like, to form the groove 57. In addition, to prevent a scattering loss due to roughness of a groove side surface, a buffer layer 58 is provided on the side surface of the groove 57. With the above structure, confinement of light is enhanced to prevent a radiation loss.
However, in the above described related art, there is a problem that, during manufacturing the device, when the distance between the pattern of the folded waveguide 53 and the pattern of the groove 57 deviates from a designed value due to a manufacturing error, a radiation loss increases and, therefore, a desired quality is not ensured.
FIG. 33 is a graph that illustrates an increase in radiation loss which occurs in response to a deviation in distance between the folded waveguide 53 and the groove 57. The ordinate axis represents a loss dB, and the abscissa axis represents a deviation μm of the groove 57. FIG. 33 illustrates an increase in radiation loss when a deviation of the groove 57 occurs vertically with respect to the folded waveguide 53.
When the deviation of the groove 57 is 0 μm, the loss is 1.4 dB, which is a desired designed value. In addition, it appears that, when the groove 57 deviates from 0 μm in a positive direction (in a direction in which the light output side waveguide of the folded waveguide 53 approaches the groove 57, and the light input side waveguide of the folded waveguide 53 leaves the groove 57), or when the groove 57 deviates from 0 μm in a negative direction (in a direction in which the light input side waveguide of the folded waveguide 53 approaches the groove 57, and the light output side waveguide of the folded waveguide 53 leaves the groove 57), a radiation loss increases in a parabolic curve.
FIG. 34 is a view that illustrates the cause of occurrence of a radiation loss when there is a deviation in distance between the folded waveguide 53 and the groove 57. When the distance between the folded waveguide 53 and the groove 57 deviates from a designed value, the modes of light do not match at a coupling portion between the straight waveguide 53a and the curved waveguide 53b (for example, mismatch occurs between the optical axis of light that propagates in the straight waveguide 53a and the optical axis of light that propagates in the curved waveguide 53b). Thus, scattering occurs and, as a result, a radiation loss increases.
Thus, with the configuration that the groove 57 is provided on the outer peripheral side of the folded waveguide 53, which is described in the related art, a radiation loss may be ideally suppressed. However, the device can not always be manufactured in accordance with a designed value at the time of manufacturing. Therefore, when the groove 57 deviates from the designed value due to a manufacturing error, a radiation loss increases and, as a result, the quality degrades. Hence, it is necessary to increase a tolerance for the manufacturing error to effectively suppress a radiation loss even when there is a manufacturing error.